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A reflection: The Effects of Atrazine on the Microbiome of the Eastern Oyster: Crassostrea virginica

By: Adrian Britt

Understanding how pesticides interact with non-target organisms is a challenge for today’s scientists. There are simply too many variables to be considered for each species that may come into contact with a pesticide once it runs off the intended application site. Thus, regulatory agencies tasked with setting pesticide thresholds rely, in part, on independent studies from the scientific community at large. Our study focused its efforts on a keystone estuarine species, Crassostrea virginica, the native eastern oyster of the Chesapeake Bay. Our lab sought to answer a vital question: How does the broad-spectrum herbicide, atrazine, effect the microbiome of the Chesapeake Bay oyster? Atrazine has already been banned from use within the E.U. because exposures to concentrations as low as 0.1 parts per billion of atrazine in surface water have been shown to adversely affect aquatic animals, causing the male gonads to produce eggs.


For this study, we relied on long-standing evidence that supports the importance of maintaining healthy populations of microbiota for the survival, homeostasis, and complete development of marine mollusks. We chose frequently detected concentrations of the chemical in surface waters to be the focus of our most recently published study, “The Effects of Atrazine on the Microbiome of the Eastern Oyster: Crassostrea virginica”. Relatively little is known about the impact that agricultural activity is having on our fallen oyster population, however, in this study we succeeded in shedding some light onto the enigmatic effects atrazine could be having on the native oyster’s microbial community, which in turn provide essential services for the oyster’s survival.


Through our study, we found that oysters which were exposed to concentrations of atrazine as low as 3µg/L saw a significant loss of key mutualistic microbial species and underwent a subsequent colonization of pathogenic bacteria. We concluded that exposure to atrazine in the Chesapeake Bay may be contributing to a significant shift in the microbiomes of juvenile oysters that reduces overall fitness and impedes natural and artificial repopulation of the oyster species within the Bay. Since the late nineteenth century, the oyster industry – including the catch, sale, shucking, packing and shipping of oysters – has contributed millions of dollars to the region’s economy.  Managing oyster populations by limiting the over-use of toxic substances will ensure that the oyster industry will become a sustainable effort. 


This is the first long term study of how ecologically relevant concentrations of atrazine affect the eastern oyster. Thus the findings this study provided will help to form the foundation for future investigations into the toxicological effects of commonly used pesticides on non-target organisms.   


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